Gabbro Akarem, Eastern Desert, Egypt: Cu-Ni-PGE mineralization in a concentrically zoned mafic-ultramafic complex

摘要

The Gabbro Akarem (Late Precambrian) intrusion is concentrically zoned with a dunite core surrounded by lherzolite-clinopyroxenite enveloped by olivine-plagioclase hornblendite and plagioclase hornblendite. Cu-Ni-PGE mineralization is closely associated with peridotite, especially in the inner, olivine-rich core (dunite pipes) where net-textured and massive sulfides (pyrrhotite, pentlandite, chalcopyrite) are found in association with Al-Mg-rich spinel and Cr-magnetite. Primary magmatic textures are well preserved; however, deformation and mobilization due to shearing are locally observed. Platinum-group minerals (PGM) documented from the deposit are: merenskyite (PdTe_2) and michenerite (PdTeBi), as well as palladian bismuthian melonite (Ni,Pd) (Te,Bi)_2. These minerals occur in intimate association with hessite (Ag_2Te) and electrum (Au_(0.65)Ag_(0.31)Bi_(0.04)) in two distinct textural positions: (1) as inclusions in pyrrhotite, pentlandite, and rarely chalcopyrite and (2) at sulfide-silicate grain boundaries and on microfractures in base-metal sulfides (BMS) and olivine associated with serpentine and secondary magnetite. Textural features suggest that PGM were exsolved from mono-sulfide solid solution over a wide range of temperatures. Late-stage, low-temperature hydrothermal solutions led to redistribution of PGF. Mineralized samples show Ni/Cu ratios ranging from 0.2 to 2 with an average of 1.0. The (Pt + Pd + Rh)/(Os + Ir + Ru) ratio is generally >6 in most samples, and Os, Ru, and Ir are below the detection limit (2 ppb). The PGE content show positive correlation with S only at low sulfur contents. The PGE patterns of Gabbro Akarem are similar to those of Alaskan-type deposits. Compared with stratiform deposits, Gabbro Akarem is depleted in PGE. The consistently low PGE contents of the mineralization and their uniform distribution in the ultramafic rocks despite the high sulfur content of the roek is attributed to rapid crystallization of sulfides in a highly dynamic environment.